If you delete a message (hit the delete key while the message is open), the next message is selected. Hit return to open it.
I find I can very efficiently read through a list of messages using the arrow keys to move up and down the list. Right arrow opens a thread, if you have threaded view on; left arrow closes the thread. Return opens the selected messages; Command-W closes the front message; Delete key deletes it.
This is especially efficient with a Smart Mailbox to list unread messages.
I never use the preview pane. This method does not use the mouse. I like it better than opening the next message automatically, since it gives more control. I can skip around if desired.
I originally thought that message navigation was the biggest deficiency in Apple Mail. Not everyone will agree, but after getting used to this keyboard-only method, I now consider message navigation -- especially using the Smart Mailbox -- to be one of Apple Mail's greatest strengths.